Polymeric materials which decompose cleanly have proven to be very valuable in molding processes such as destructive foam casting, sometimes referred to as lost plastic pattern casting, and sintering of ceramic or metallic powders. For example, U.S. Pat. No. 3,351,123 discloses an evaporative pattern casting process using a mold of foamed thermoplastic resin with a refractory coating. Synthetic resins disclosed as useful in this process include polymers of cellulose ethers and esters, olefins, vinyl esters of carboxylic acids, vinyl ethers, methacrylic acid esters of alcohols containing up to 18 carbon atoms, vinyl aromatic compounds, unsaturated carboxylic acids, and the like. In general, polymers of monomeric materials containing a vinylidene group are useful. From the group of resins disclosed by this patent, only two have been attractive commercially, these being polystyrene and poly(methyl methacrylate). Polystyrene, however, does not decompose cleanly, which is typical for polymers containing aromatic structural units. On decomposing, these polymers yield a carbonaceous residue which reduces the quality of the metal casting and is unacceptable where high quality castings are required. Poly(methyl methacrylate) decomposes without carbonaceous residue. This polymer, however, releases large quantities of gas upon exposure to the molten metal and presents a problem in the build-up of unacceptable pressure which results in casting flaws.
U.S. Pat. No. 3,942,583, Baur, (1976) also discloses using lost plastic patterns for metal casting molds built up from plastic plate or sheet material bonded to an internal support frame. The plastic plate or sheet materials can be made from foamed plastics, such as foams of polyurethane, phenolic resin, polystyrene, cellulose acetate, polyvinyl chloride, polyethylene or poly(vinyl acetate).
U.S. Pat. No. 4,633,929, Santangelo, et al., (1987) discloses that poly(alkylene carbonates) formed from carbon dioxide and oxirane compounds can be used for evaporative pattern casting and on decomposition yield no carbonaceous deposit in contrast to polystyrene. Poly(propylene carbonate) and other poly(alkylene carbonates) having molecular weights in the range of 5,000 to 2,000,000 and higher can be foamed with blowing agents and used in such casting operations.
Decomposable polymers are also valuable in service as binders for either ceramic or metallic powders which are then shaped and fused by sintering at high temperatures. A description of the utility of the polymeric binders for making parts of ceramic powders is described in T. Morse, "Handbook of Organic Additives for Use in Ceramic Body Formulation", Montana Energy and MHD Research and Development Institute, Butte, Montana, (1979). In addition to binders, other additives are used to serve as lubricants, dispersants or antifoams. The binder/lubricant system is baked out of the ceramic shape during the bisque firing step. Binders having a rapid burn-off require a long, slow bisque firing. Binders disclosed include poly(vinyl alcohol), acrylic resins, poly(vinyl butyral) and waxes.
U.S. Pat. Nos. 3,585,168 (1971) and 3,953,383 (1976) to Inoue, et al., describe the preparation of poly(propylene carbonate) by the copolymerization of propylene oxide and carbon dioxide. A detailed discussion concerning the preparation of poly(propylene carbonates) as well as other poly(alkylene carbonates) is given by Inoue, Higashi and Yamazaki, "Synthesis of Macromolecules from Carbon Dioxide", Organic and Bio-Organic Chemistry of Carbon Dioxide, Chapter 4, John Wiley & Sons, New York (1982).
F. Rodriquez, Principles of Polymer Systems, pages 407-411 McGraw-Hill Book Company, New York (1970), describes the polymerization of methyl methacrylate, a well known monomer which has been used commercially to form polymers for several decades.
U.S. Pat. No. 4,698,390, Robeson, et al., (1987) describes a compatible blend of a vinyl chloride derived polymer and a polycarbonate having repeating units derived from bis-(3,5-dimethyl,4-hydroxyphenyl) sulfone. The vinyl chloride polymer can be a copolymer of at least 50 wt. % vinyl chloride and methyl methacrylate. The polycarbonates disclosed are quite different from poly(propylene carbonate) and do not suggest blends of such polymers with poly(methyl methacrylate).
Japanese Pat. No. Application SHO 62/21753, Inoue, et al. (1987) discloses using poly(alkylene carbonate), preferably poly(ethylene carbonate) and poly(propylene carbonate), as a binder for molding ceramic or metallic powders. Additives such as plasticizers, lubricants and other binders, can be used as long as the purpose is not adversely affected. Other binders listed are polyethylene, polypropylene, polybutene, poly(methyl methacrylate), polystyrene, ethylene-vinyl acetate copolymer, as well as their low molecular weight compounds, natural waxes and the like. There is no recognition of any benefit in combining poly(propylene carbonate) and poly(methyl methacrylate) specifically in any proportions for use as a binder nor is there any suggestion of use in evaporative foam casting.